Hydrophilic polyurethane polymers derived from a mdi-based isocyanate-terminated prepolymer

ABSTRACT

The present invention relates an MDI-based isocyanate-terminated prepolymer suitable for preparing a hydrophilic polyurethane polymer. The MDI-based composition comprises a prepolymer obtained by reacting isocyanate mixture comprising 2,4′-methylene diphenylisocyanate (MDI) and 4,4′-methylene diphenylisocyanate in a molar ratio of from 25:75 to 80:20 with a polyether polyol having an oxyethylene content of at least 30 percent by weight.

The present invention relates an MDI-based isocyanate-terminatedprepolymer suitable for preparing a hydrophilic polyurethane polymer.

Hydrophilic polyurethane foams can be prepared by a process in which ahydrophilic prepolymer having isocyanate end groups is mixed and reactedwith water. U.S. Pat. Nos. 3,861,993 and 3,889,417 disclose ahydrophilic polyurethane foam which is formed by mixing and reactingwith water an isocyanate capped polyoxyethylene glycol prepolymer usinga molar ratio of H₂O to NCO groups in the prepolymer of 6.5 to 390:1.

A particular family of polyurethane prepolymers, derived from methylenediphenylisocyanate (MDI) and the aqueous two-stage process foamsproduced therefrom, are disclosed in U.S. Pat. No. 4,365,025. Anisocyanate-containing prepolymer in which the isocyanate is a mixture ofMDI and polymeric forms of MDI is foamed by mixing it with anapproximately equal amount of water. The resultant flexible foams arecharacterized by greater hydrolytic stability than those foamed fromtoluene diisocyanate (TDI) prepolymers. Often other hydrophilicmaterials, such as fibers (See, for example, U.S. Pat. No. 4,127,516) orsuperabsorbent polymers (See, for example, U.S. Pat. No. 5,064,653 andU.S. Pat. No. 6,034,149) or particles (See, for example, U.S. Pat. No.3,224,889) may be incorporated into the foam structures to improvephysical properties including, for example, wet out rates of such foams.

U.S. Pat. No. 4,314,034 discloses a polyurethane foamed sponge formedfrom the combination of a hydrophilic oxyalkylene polyol capped withisocyanate groups and 1 to 30 percent by weight of a polymericpolyisocyanate, such as a polymeric MDI, sold by The Dow ChemicalCompany under the trademark PAPI. The prepolymers are foamed in thepresence of water, reinforcing fibers, surfactants, a thickening agentand up to 30 percent by weight of diatomaceous earth. With thediatomaceous earth and PAPI polymeric MDI the foam sponge has what isdescribed as rapid wet out and improved wet strength.

U.S. Pat. No. 5,650,450 discloses a hydrophilic foam prepared byreacting a MDI/polymeric MDI-based isocyanate capped prepolymer with anaqueous component comprising a selected silicone glycol copolymer liquidsurfactant.

Despite an extensive knowledge concerning the preparation of hydrophilicpolyurethane foams based on MDI-based prepolymers, TDI based prepolymersare still extensively used in the industry. The preferred use ofTDI-based prepolymers is based on the frequent inferior performance ofMDI-based prepolymers as compared to the TDI prepolymer. However, due toever stricter health and safety regulations surrounding TDI and TDIbased adducts, there is a need to provide alternative MDI-basedprepolymers which when used in the preparation of hydrophilicpolyurethane foam provide for a competitive performance.

It has now been discovered that use of MDI-based prepolymers based on anisocyanate having an elevated 2,4′-MDI content, when preparinghydrophilic polymers for foamed and non-foamed products, can satisfymany unmet needs. The MDI-based prepolymers and isocyanate compositionsas presented in the above-noted publications are generally based oncompositions that have a minimal 2,4-MDI content and typically at most a10 percent 2,4-MDI content.

In a first aspect, this invention relates to a hydrophilic polyurethanepolymer prepared by bringing together water and an isocyanate-terminatedprepolymer wherein the prepolymer is the reaction product of:

-   -   a) a polyether polyol composition having a nominal hydroxyl        functionality of from 1.6 to 8, a molecular weight of from 1000        to 12000 and having at least 30 percent by weight of oxyethylene        groups,    -   b) with an isocyanate mixture that contains methylene        diphenylisocyanate (MDI) in at least about 60 weight percent of        the total isocyanate present and wherein the MDI comprises the        2,4′- and 4,4′-methylene diphenylisocyanate isomer in a molar        ratio of from 25:75 to 80:20;        the prepolymer having a free NCO content of from 1 to less than        15 percent by weight.

In a second aspect, this invention relates to an isocyanate-terminatedprepolymer composition that has a free isocyanate content of less than15 percent and is the reaction product of:

-   -   a) a polyether polyol composition having a nominal hydroxyl        functionality of from 1.6 to 8, a molecular weight of from 1000        to 12000 and having at least 30 percent by weight of oxyethylene        groups;    -   b) with a stoichiometric excess of an isocyanate mixture that        contains methylene diphenylisocyanate (MDI) in at least about 60        weight percent of the total isocyanate present and wherein the        MDI comprises the 2,4′- and 4,4′-methylene diphenylisocyanate        isomer in a molar ratio of from 25:75 to 80:20.

In a third aspect, this invention relates to process for making ahydrophilic polyurethane foam comprising mixing together an aqueousphase and an isocyanate-terminated prepolymer composition that is thereaction product of:

-   -   a) a polyether polyol having a nominal hydroxyl functionality of        from 1.6 to 8, a molecular weight of from 1000 to 12000 and        having at least 30 percent by weight of oxyethylene groups; with        a stoichiometric excess of    -   b) an isocyanate mixture that contains methylene        diphenylisocyanate (MDI) in at least about 60 weight percent of        the total isocyanate present and wherein the MDI comprises the        2,4′- and 4,4′-methylene diphenylisocyanate isomer in a molar        ratio of from 25:75 to 80:20        wherein the isocyanate-terminated prepolymer composition and        aqueous phase are present in a parts by weight ratio of from        100:10 to 100:500 and the proviso that the free NCO content of        the prepolymers is less than 15 percent.

In a fourth aspect, this invention relates to process for making ahydrophilic polyurethane gel comprising mixing together an aqueous phaseand an isocyanate-terminated prepolymer composition that is the reactionproduct of:

-   -   a) a polyether polyol having a nominal hydroxyl functionality of        from 1.6 to 8, a molecular weight of from 1000 to 12000 and        having at least 30 percent by weight of oxyethylene groups; with        a stoichiometric excess of    -   b) an isocyanate mixture that contains methylene        diphenylisocyanate (MDI) in at least about 60 weight percent of        the total isocyanate present and wherein the MDI comprises the        2,4′- and 4,4′-methylene diphenylisocyanate isomer in a molar        ratio of from 25:75 to 80:20

wherein the isocyanate-terminated prepolymer composition and aqueousphase are present in a parts by weight ratio of from 100:900 to100:5000, with the proviso that the NCO content of the prepolymer isless than 15 percent.

The hydrophilic polyurethane polymer of this present invention isprepared by bringing and reacting together an aqueous phase with anisocyanate-terminated prepolymer. The resulting polymer can have a foamor gel structure. The isocyanate-terminated prepolymer generally has anaverage isocyanate content of greater than 1 and generally less than 15weight percent. Preferably, the polymer has an average isocyanatecontent of from 1 to 14, more preferably from 2 to 12, and yet morepreferably from 3 to 10 weight percent. Typically when intending toprepare a gel, the isocyanate-terminated prepolymer advantageously hasan isocyanate content of from 1 to 6, preferably from 2 to 5 weightpercent. The isocyanate-terminated prepolymers of higher isocyanatecontent are generally preferred when intending to prepare foam.Polyisocyanate can be back blended into the prepolymer to reach thedesired free NCO content.

The prepolymer is the reaction product of a polyether polyol with astoichiometric excess of an isocyanate mixture that contains methylenediphenylisocyanate (MDI) isomers in at least about 60 weight percent oftotal isocyanate present and wherein the MDI comprises the 2,4′- and4,4′-methylene diphenylisocyanate isomers in a molar ratio of from 25:75to 80:20, preferably from 40:60 to 80:20, more preferably in from 45:55to 70:30. The balance of the isocyanate mixture when not methylenediphenylisocyanate can comprise toluene diisocyanate, hexamethylenediisocyanate, isophorone diisocyanate, polymethylenepolyphenylisocyanate, carbodiimide or allophonate or uretonimine adductsof methylene diphenylisocyanate and mixtures thereof Preferredisocyanates to make up the balance of the composition are polymethylenepolyphenylisocyanate, carbodiimide or allophonate or uretonimine adductsof methylene diphenylisocyanate. In a particularly preferred embodiment,the isocyanate mixture used to prepare the prepolymer consistsessentially of 2,4′- and 4,4′-methylene diphenylisocyanate isomers in amolar ratio of from 25:75 to 80:20, preferably from 40:60 to 80:20; morepreferably from 45:55 to 70:30. Preferably, the isocyanate mixturecontains greater than 40 percent by weight of the 2,4-MDI isomer. Thecombination of a low NCO content and increased 2,4-MDI isomer in theprepolymer, when reacted with water, unexpectedly give a foam havinghigh hydrophilicity and good properties in terms on foam density andflexibility.

The polyether polyol composition used to prepare the prepolymercomprises one or more polyether polyols and has an average nominalhydroxyl functionality of from 1.6 to 8, a molecular weight of from 1000to 12000 and having at least 30 percent, preferably at least 40 percent,and more preferably at least 50 percent, and yet more preferably atleast 65 percent by weight of oxyethylene groups. By the term “nominal”,it is meant the average hydroxyl functionality of the composition withthe assumption that the average functionality of the polyol is identicalto that average functionality (active hydrogen atoms per molecule) ofthe initiator as used in the preparation of the polyol; in practice itmay be somewhat less due to presence of some terminal unsaturation. Bythe term “molecular weight”, it is understood the theoretical molecularweight expressed in Daltons and being calculated with consideration tothe molecular weights and amounts of reactants from which the polyol isprepared. In a preferred embodiment, the polyol composition has anominal functionality of from 1.6 to 6, more preferably from 1.6 to 4,and yet more preferably from 1.6 to 2.5. Similarly, the preferredmolecular weight for the polyol composition is from 1000 to 8000, morepreferably from 1000 to 5000, and yet more preferably from 1500 to 3500.Combinations of polyether polyols, including monohydroxyl substances andlow molecular weight diol and triol substances or amines, of varyingfunctionality and oxyethylene content can be used but then in this eventthe average functionality, molecular weight and oxyethylene content ofsuch combination will be as mentioned above. It is also to advantagethat the oxyethylene content of the individual polyols be randomlydistributed through out the molecule.

In a particularly preferred embodiment of this invention: the polyetherpolyol composition used to prepare the prepolymer is a polyether polyolhaving a nominal hydroxyl functionality of from 1.6 to 2.5, a molecularweight of from 1500 to 3500 and having at least 65 percent by weight ofoxyethylene groups.

In a particularly preferred embodiment of this invention theisocyanate-terminated prepolymer prepared and subsequently used tomanufacture a hydrophilic polymer has an isocyanate content of from 2 to15 weight percent and is obtained by reacting an isocyanate mixture thatconsists essentially of the 2,4′- and 4,4′-methylene diphenylisocyanateisomers in a molar ratio of from 45: 55 to 70:30; with a polyolcomposition comprising a polyether polyol having a nominal hydroxylfunctionality of from 1.6 to 2.5, a molecular weight of from 1500 to3500 and having at least 65 percent by weight of oxyethylene groups.

The polyoisocyanates and polyols used in the present invention arecommercially available or can be produced using standard processes knownto those skilled in the art. In general the polyether polyol isobtainted in conventional manner by reacting ethylene oxide and/orpropylene oxide simultaneously and/or sequentially in any order with aninitiator having 2 to 8 active hydrogen atoms. Such initiators includewater, ethylene glycol, propylene glycol, butanediol, glycerol,trimethyol propane, ethylene diamine, triethanolamine, sucrose andsorbitol.

The prepolymer is prepared in a conventional way by combining thediisocyanate and the polyol at 20-100° C. and if desired, in thepresence of urethane-forming catalyst, such as a tertiary amine or tincompound. The relative amounts of the diisocyanate and the polyol arechosed in such a way as to arrive at the desired free NCO content of thefinal product. In general the equivalent amount of diisocyanate will behigher than the equivalent amount of the polyol. The preparation of theprepolymer is a routine operation for those skilled in the art.

In making a polyurethane polymer, the ratio of the amount ofisocyanate-terminated prepolymer to the aqueous mixture can vary over awide range depending on the target density of the resulting polymer andits associated physical parameters; and also on the isocyanate contentof the composition. When intending to prepare a hydrophilic foam,typically 100 parts by weight of the isocyanate-terminated prepolymerwill be mixed and reacted with from 10 to 500 parts by weight of anaqueous phase, preferably 50 to 300, and more preferably with from 50 to150 parts by weight of the aqueous phase. Conversely when intending toprepare a hydrophilic gel, 100 parts by weight of theisocyanate-terminated prepolymer will be mixed and reacted with from5000 to 900 and preferably from 3500 to 1500 parts by weight of anaqueous phase. Polymers having utility as a binder may also be preparedin a similar manner as described herein; generally the relative amountsof isocyanate-terminated prepolymer to aqueous phase will be between theranges noted for the foam and gel applications. A preferred method offoaming the isocyanate-terminated prepolymer comprising prepolymer is tobring the aqueous phase, for example, a 2 percent solution of thesurfactant to a temperature of from 5° C. to 50° C. and introduce tothis the isocyanate-terminated prepolymer. The resulting mixture is thenbrought to the mold or pour area, dispensed and allowed to react out.

When intended to use the isocyanate-terminated prepolymer in themanufacture of a hydrophilic polyurethane foam, it is found advantageousto incorporate a crosslinking agent within the prepolymer in contrast tohaving it present in the hydroxyl composition to be reacted with theisocyanate/prepolymer composition. Introduction of the cross-linkingagent in this manner facilitates preparation of foam with attractivemechanical properties. Representative of crosslinkers suitable forincorporation into the prepolymer are low molecular weight polyolstypically having an average hydroxyl functionality of from 3 to 4, orlow molecular weight amines having typically 3 or 4 amine moieties.Illustrative and preferred are glycerine, trimethylolpropane and lowmolecular weight alkotylated derivatives thereof Ethylene diamine isalso commonly used although it is a less preferred amine cross-linkingagent for use with the present invention. Such cross-linking agent maybe present in an amount of from 0.1 to 5, preferably from 0.5 to 3 andmore preferably from 1 to 3 percent of the total amount by weight ofpolyether polyol, cross-linking agent and optional viscosity modifier tobe reacted with the isocyanate.

Hydrophilic polyurethane foam is prepared by contacting under thereaction conditions the isocyanate-terminated prepolymer with an aqueousphase. The aqueous phase comprises essentially water and, as might berequired, minor amounts of surfactant, catalyst, or a thickening agent.While it is possible to prepare hydrophilic foam in the absence ofsurfactant it is advantageous to have present a surfactant. Surfactantsare chosen to give a foam with a good appearance of cell structure andsize and to minimize collapse and or foam deformations, such as forexample splitting. Examples of preferred surfactants are the blockcopolymers of oxyethylene and oxypropylene such as the Pluronic Polyolsurfactants manufactured by BASF. Generally preferred are the non-ionicsurfactants such as available under the Pluronic trade name and includethe designated products L-62, L-72, 1-92, P-75 or P-85. Othersurfactants equivalent in nature or preformance may be used in place ofthe mentioned substances. Surfactants typically will be present in theaqueous phase in an amount of from 0.5 to 4, preferably from 0.75 to3.0, parts by weight per 100 parts by weight of the total aqueous phaseincluding surfactant;

Hydrophilic foams frequently can be prepared in the absence of acatalyst. However if required, a catalyst may be incorporated into theisocyanate-terminated prepolymer/aqueous mixture by premixing with theaqueous mixture or alternatively with the isocyanate-terminatedprepolymer but then in this instance only immediately before it use inreaction with the aqueous mixture. When required, the catalyst is addedin an amount to modify the curing time of the reaction product andfacilitate in attaining the desired physical attributes of the foam;Suitable common catalysts are substances generally known in the art forpromoting the reaction of isocyanate with a polyol and include basicsubstances such as sodium bicarbonate or the tertiary amines andorganometallic compounds. Illustrative of suitable catalysts includen-methyl morpholine, n-ethyl morpholine, trimethylamine, triethylamine,tetramethyl butane diamine, triethylenediamaine,dimethylaminoethanolamine, benzylidimethylamine, dibutyl tin dilaurateand stannous octoate.

Thickening agents may be present when it is desired to control theviscosity of the aqueous phase and facilitate the transportation anddistribution of, for example, fillers or fibers. Exemplary of typicalfillers includes clays, diatomaceous earth, calcium carbonate, andmineral fibers such as wallastonite; aqueous latexes such as for examplea styrene-butadiene. Examples of thickening agents are natural productssuch as xanthan gums, or chemical agents such as polyacrylamide polymersand gels as sold by The Dow Chemical Company. Other additives which mayalso be present include mixing aids and notably emulsifiers.

The aqueous phase may also be used to introduce to other substances,such as fatty oils and functional additives, besides fibers and fillerswhen desiring to modify physical properties of the resulting polymer.Also present can be fragrances or perfumes or other such substances thatcan be detected by scent should this be required for the endapplication. If the end application requires a polymer that has somephysiological active properties, the aqueous phase can also be used tointroduce active molecules such as for example, pesticides,insecticides, herbicides, attractants, pheromones, growth promoting orregulating substances or plant or animal nutrients. If the resultingpolymer is to be used in end applications where electrical orluminescent properties are required, the aqueous mixture may be used tointroduce electrolytes so as to render the polymer electro-conductive,or fluorescent or phosphorescent additives so as to render the polymerluminescent. While generally such additional substances are introducedvia the aqueous phase, the isocyanate-terminated prepolymer can also beutilized in the same manner when no adverse reactions or processconditions prevail.

Additives, such as those disclosed above, for the production of gelsbased on the prepolymers of the present invention can be used in theproduction of a gel.

Foams produced by the prepolymers of the present invention are usefulfor safety applications, such as ear plugs; cosmetics, such as facialsponges; wound dressing, such as bandages; clothing, such as shoulderpads, etc. Gels produced by the prepolymers of the present invention areparticularly useful in support applications, such as furniture; use inair fresheners and in biosenors, that is matrix for enzymeimmobiliation.

The following examples are given to illustrate the invention and shouldnot be interpreted as limiting it in any way. Unless stated otherwise,all parts and percentages are by weight.

EXAMPLE 1

An isocyanate-terminated prepolymer is prepared by reacting 16.9 partsof a 50:50 mixture of 2,4′-methylene diphenylisocyanate and4,4′-methylene diphenylisocyanate with 83 parts by weight of a polyetherpolyol, NIXOLEN VS 40 available from Enichem and understood to be apolyol with a nominal functionality of 2; a molecular weight of about2600; and a randomly distributed oxyethylene content of about 75 weightpercent. The isocyanate is introduced to a reaction vessel and broughtto a temperature of about 70° C.; the polyol is then added inincremental amounts over a period of about 2 hours. The resultingmixture is stirred continuously until a constant isocyanate content isobserved. The resulting isocyanate-terminated prepolymer is observed tohave an isocyanate content of 3.2 percent and a viscosity of 14,400cpsat 23° C.

EXAMPLE 2

The procedure of Example 1 is used to prepare an isocyanate-terminatedprepolymer by reacting 25 parts of a 50:50 mixture of 2,4′-methylenediphenylisocyanate and 4,4′-methylene diphenylisocyanate with 74.9 partsby weight of a polyether polyol, NIXOLEN VS 40. The resultingisocyanate-terminated prepolymer is observed to have an isocyanatecontent of 6.3 percent and a viscosity of 6000 cps at 23° C.

EXAMPLE 3

The procedure of Example 1 is used to prepare an isocyanate-terminatedprepolymer by reacting 34.6 parts of a 50:50 mixture of 2,4′-methylenediphenylisocyanate and 4,4′-methylene diphenylisocyanate with 65.4 partsby weight of a polyether polyol, NIXOLEN VS 40. The resultingisocyanate-terminated prepolymer is observed to have an isocyanatecontent of 9.3 percent and a viscosity of 3500 cps at 23° C.

EXAMPLE 4

The procedure of Example 1 is used to prepare an isocyanate-terminatedprepolymer by reacting 26.7 parts of a 50:50 mixture of 2,4′-methylenediphenylisocyanate and 4,4′-methylene diphenylisocyanate with 73.3 partsby weight of a polyether polyol, POLY 22-56 available from ARCH andunderstood to be a diol having an average molecular weight of about 2000and an oxyethylene content of about 80 percent. The resultingisocyanate-terminated prepolymer is observed to have an isocyanatecontent of 5.9. percent and a viscosity of 12000 cps at 23° C.

EXAMPLE 5

The procedure of Example 1 is used to prepare an isocyanate-terminatedprepolymer by reacting 26,7 parts of a 50:50 mixture of 2,4′-methylenediphenylisocyanate and 4,4′-methylene diphenylisocyanate with 73,3 partsby weight of a polyether polyol, POLY 22-56 available from ARCH andunderstood to be a diol having an average molecular weight of about 2000and an oxyethylene content of about 80 percent. The resultingisocyanate-terminated prepolymer is observed to have an isocyanatecontent of 9.1 percent and a viscosity of 12,000 cps at 23° C.

EXAMPLE 6

Hydrophilic polyurethane foams are prepared by reacting the indicatedIsocyanate-terminated prepolymer with an aqueous phase “A1

” or “A2”. The aqueous phase “A1” comprises the surfactant PLURONIC PE6800, available from BASF, at a 2wt percent concentration. The aqueousphase A2 comprises the surfactant EMULGADE 1000 NI, available fromHenkel, understood to be a blend of a fatty alcohol derivatives; also ata concentration of 2 wt percent. Physical properties of the resultingfoams as reported in the following table are observed according to thefollowing procedures; density by DIN 53420; tensile properties andelongation by DIN 53571A; and tear properties by DIN 53515. Theisocyanate-terminated prepolymers HYPOL 2000 and HYPOL 3000 arecommercially available TDI-based compositions from The Dow ChemicalCompany. As can be seen from the reported data the hydrophilicpolyurethane foam prepared from the MDI-based isocyanate exhibitsequivalent or better performance than the TDI-based hydrophilicpolyurethane foam. Density Tensile Tensile Elongation Elongation TearTear NCO Aqueous Dry foam Dry Wet Dry Wet Dry Wet System percent phase[kg/m³] [N/mm²] [N/mm²] [percent] [percent] [N/mm] [N/mm] Hypol 2000 6.6A1 105 0.165 0.032 234 111 0.74 0.26 Hypol 2000 6.6 A2 104 0.088 0.031246 99 0.51 0.10 NCO of Example 2 6.3 A1 112 0.094 0.063 298 153 1.280.97 NCO of Example 2 6.3 A2 121 0.115 0.065 267 127 1.11 Not measuredNCO of Example 4 5.9 A1 109 0.095 0.054 269 154 1.10 0.96 NCO of Example4 5.9 A2 106 0.073 0.058 264 183 1.19 0.70 Hypol 3000 9.4 A1 88 0.0940.044 176 92 0.73 0.25 Hypol 3000 9.4 A2 93 0.114 0.037 163 68 0.51 0.15NCO of Example 3 9.3 A1 67 0.056 0.041 208 101 0.78 0.47 NCO of Example3 9.3 A2 72 0.085 0.066 298 155 0.75 0.52 NCO of Example 5 9.1 A1 760.080 0.040 256 113 0.915 0.47 NCO of Example 5 9.1 A2 75 0.076 0.071293 215 0.803 0.52

EXAMPLE 7 (COMPARATIVE)

The procedure of Example 1 is used to prepare an isocyanate-terminatedprepolymer by reacting 41.8 parts of a 2:98 mixture of 2,4′-methylenediphenylisocyanate and 4,4′-methylene diphenylisocyanate with 55.7 partsby weight of a polyether polyol, POLY 22-56 available from ARCH andunderstood to be a diol having an average molecular weight of about 2000and an oxyethylene content of about 80 percent and with 2.5 parts oftrimethylolpropane. The resulting isocyanate-terminated prepolymer isobserved to have an isocyanate content of 9.1 percent and a viscosity of19,000 cps at 25° C.

EXAMPLE 8

The procedure of Example 1 is used to prepare an isocyanate-terminatedprepolymer by reacting 41.8 parts of a 50:50 mixture of 2,4′-methylenediphenylisocyanate and 4,4′-methylene diphenylisocyanate with 55.7 partsby weight of a polyether polyol, POLY 22-56 available from ARCH andunderstood to be a diol having an average molecular weight of about 2000and an oxyethylene content of about 80 percent and with 2.5 parts oftrimethylolpropane. The resulting isocyanate-terminated prepolymer isobserved to have an isocyanate content of 9.1 percent and a viscosity of18,000 cps at 25° C. With reference to Example 7 it is observed thatisocyanate-terminated prepolymers prepared with elevated amounts of2,4′-MDI exhibit a lower viscosity.

EXAMPLE 9 (COMPARATIVE)

The procedure of Example 1 is used to prepare an isocyanate-terminatedprepolymer by reacting 47.5 parts of a 2:98 mixture of 2,4′-methylenediphenylisocyanate and 4,4′-methylene diphenylisocyanate with 50 partsby weight of a polyoxyethylene glycol having a molecular weight of 1000and with 2.5 parts of trimethylolpropane. The resultingisocyanate-terminated prepolymer is observed to have an isocyanatecontent of 9.2 percent and a viscosity of 58,700 cps at 25° C.

EXAMPLE 10

The procedure of Example 1 is used to prepare an isocyanate-terminatedprepolymer by reacting 47.5 parts of a 50:50 mixture of 2,4′-methylenediphenylisocyanate and 4,4′-methylene diphenylisocyanate with 50 partsby weight of a polyoxyethylene glycol having a molecular weight of 1000and with 2.5 parts of trimethylolpropane. The resultingisocyanate-terminated prepolymer is observed to have an isocyanatecontent of 9.2. percent and a viscosity of 44,500 cps at 25° C. Withreference to Example 9 it is observed that isocyanate-terminatedprepolymers prepared with elevated amounts of 2,4′-MDI exhibit a lowerviscosity.

EXAMPLE 11

The procedure of Example 1 is used to prepare an isocyanate-terminatedprepolymer by reacting 44 parts of a 50:50 mixture of 2,4′-methylenediphenylisocyanate and 4,4′-methylene diphenylisocyanate with a polyolmixture containing (I) 25.5 parts by weight of a polyoxyethylene glycolhaving a molecular weight of 1000 polyoxyethylene glycol having amolecular weight of 1000 (ii) 25.5 parts by weight of a glycerineinitiated polyoxyethylene-oxypropylene polyol having a molecular weightof about 7800 and an oxyethylene content of about 70 percent; and (iii)5 parts by weight of dipropylene glycol The resultingisocyanate-terminated prepolymer is observed to have an isocyanatecontent of 8.9.percent and a viscosity of 32,600 cps at 25° C.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of this specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

1) A hydrophilic polyurethane polymer prepared by bringing togetherwater and an isocyanate-terminated prepolymer wherein the prepolymer isthe reaction product of: a) a polyether polyol composition having anominal hydroxyl functionality of from 1.6 to 8, a molecular weight offrom 1000 to 12000 and having at least 30 percent by weight ofoxyethylene groups, b) with an isocyanate mixture that containsmethylene diphenylisocyanate (MDI) in at least about 60 weight percentof the total isocyanate present and wherein the MDI comprises the 2,4′-and 4,4′-methylene diphenylisocyanate isomer in a molar ratio of from25:75 to 80:20; the prepolymer having a free NCO content of from 1 toless than 15 percent by weight. 2) The hydrophilic polyurethane polymerof claim 1 obtained from an isocyanate-terminated prepolymer which hasan isocyanate content of from 1 to 14 weight percent. 3) The hydrophilicpolyurethane polymer of claim 2 obtained from an isocyanate-terminatedprepolymer which has an isocyanate content of from 2 to 12 weightpercent. 4) The hydrophilic polyurethane polymer of claim 1 wherein theisocyanate mixture consists essentially of the 2,4′- and 4,4′-methylenediphenylisocyanate isomers in a molar ratio of from 40:60 to 80:20. 5)The hydrophilic polyurethane polymer of claim 4 wherein the isocyanatemixture consists essentially of the 2,4′- and 4,4′-methylenediphenylisocyanate isomers in a molar ratio of from 45:55 to 70:30. 6)The hydrophilic polyurethane polymer of claim 1 wherein the remainingbalance of the isocyanate-terminated prepolymer comprises toluenediisocyanate, hexamethylene diisocyanate, isophorone diisocyanate,polymethylene polyphenylisocyanate, carbodiimide or allophonate adductsof methylene diphenylisocyanate. 7) The hydrophilic polyurethane polymerof claim 1 wherein the polyether polyol polyether polyol has a nominalhydroxyl functionality of from 1.6 to 4, a molecular weight of from 1000to 8000 and at least 50 percent by weight of oxyethylene groups. 8) Anisocyanate-terminated prepolymer composition that has an isocyanatecontent of less than 15 weight percent and is the reaction product of:a) a polyether polyol composition having a nominal hydroxylfunctionality of from 1.6 to 8, a molecular weight of from 1000 to 12000and having at least 30 percent by weight of oxyethylene groups; b) witha stoichiometric excess of an isocyanate mixture that contains methylenediphenylisocyanate (MDI) in at least about 60 weight percent of thetotal isocyanate present and wherein the MDI comprises the 2,4′- and4,4′-methylene diphenylisocyanate isomer in a molar ratio of from 25:75to 80:20. 9) An isocyanate-terminated prepolymer composition that has anisocyanate content of from 2 to 12 weight percent and is the reactionproduct of: a) a polyether polyol having a nominal hydroxylfunctionality of from 1.6 to 4, a molecular weight of from 1000 to 5000and having at least 65 percent by weight of oxyethylene groups; with astoichiometric excess of b) an isocyanate mixture that consistsessentially of methylene diphenylisocyanate (MDI) wherein the MDIcomprises the 2,4′- and 4,4′-methylene diphenylisocyanate isomer in amolar ratio of from 40:60 to 60:40. 10) A process for making ahydrophilic polyurethane foam comprising mixing together an aqueousphase with an isocyanate-terminated prepolymer composition that is thereaction product of: a) a polyether polyol composition having a nominalhydroxyl functionality of from 1.6 to 8, a molecular weight of from 1000to 12000 and having at least 30 percent by weight of oxyethylene groups;with a stoichiometric excess of b) an isocyanate mixture that containsmethylene diphenylisocyanate (MDI) in at least about 60 weight percentof total isocyanate present and wherein the MDI comprises the 2,4′- and4,4′-methylene diphenylisocyanate isomer in a molar ratio of from 25:75to 80:20, wherein the isocyanate-terminated prepolymer composition andaqueous phase are present in a parts by weight ratio of from 100:10 to100:500 and the proviso that the free NCO content of the prepolymer isless than 15 weight percent. 11) The process of claim 10 wherein theaqueous phase consists essentially of water and from 0.1 to 4 parts byweight of surfactant per 100 parts by weight of the total aqueous phaseincluding surfactant. 12) A process for making a hydrophilicpolyurethane gel comprising mixing together an aqueous phase with anisocyanate-terminated prepolymer composition that is the reactionproduct of: a) a polyether polyol composition having a nominal hydroxylfunctionality of from 1.6 to 8, a molecular weight of from 1000 to 12000and having at least 30 percent by weight of oxyethylene groups; with astoichiometric excess of b) an isocyanate mixture that containsmethylene diphenylisocyanate (MDI) in at least about 60 weight percentof total isocyanate present and wherein the MDI comprises the 2,4′- and4,4′-methylene diphenylisocyanate isomer in a molar ratio of from 25:75to 80:20, wherein the isocyanate-terminated prepolymer composition andaqueous phase are present in a parts by weight ratio of from 100:900 to100:5000 and the proviso that the free NCO content of the prepolymer isless than 15 weight percent.